Abstract:

It was found that kojic acid, which is used in cosmetics for its excellent whitening effect, inhibits catecholase activity of tyrosinase in a non-classical manner. A decrease in the initial velocity to a steady-state inhibited velocity can be observed over a few minutes. This time-dependence, which is unaltered by prior incubation of the enzyme with the inhibitor, is consistent with a first-order transition. The kinetic data obtained correspond to those for a postulated mechanism that involves the rapid formation of an enzyme inhibitor complex that subsequently undergoes a relatively slow reversible reaction. Kinetic parameters characterizing this type of inhibition were evaluated by means of nonlinear regression of product accumulation curves.

Abstract:

The mechanism by which ultraviolet radiation induces melanogenesis in epidermal melanocytes is unknown. Previous observations that in cultured human melanocytes 1-oleoyl-2-acetylglycerol augmented both basal and ultraviolet radiation-induced melanogenesis, suggested that the responses were mediated via protein kinase C. However, paradoxically the phorbol ester TPA was without effect. Therefore, the present study has examined the involvement of protein kinase C in melanogenesis. Analysis of the isozyme profile of human melanocytes revealed the presence of protein kinase C alpha, beta I, epsilon and zeta but not the isozyme eta. Following exposure to 500 nM TPA for 24 hours, isozymes alpha, beta I and epsilon were downregulated, but zeta was unaffected. Similar isozyme profiles were observed in S91 and SKMEL3 melanoma cells. The melanogenic responses to 1-oleoyl-2-acetylglycerol and ultraviolet radiation were unaffected by inhibition of protein kinase C with Ro31-8220, or ablation by downregulation with 500 nM TPA, in human melanocytes and melanoma cells. 1-Oleoyl-2-acetylglycerol had no effect on protein kinase C activity in human melanocytes, as measured by rapid phosphorylation of the 80 kDa protein myristoylated alanine-rich C kinase substrate (MARCKS). Ultraviolet radiation induced a small increase in MARCKS protein phosphorylation but this effect was inhibited by pretreatment for 24 hours with 500 nM TPA, which had no effect on ultraviolet-induced melanogenesis. Overall, these findings indicate that 1-oleoyl-2-acetylglycerol and ultraviolet radiation activate melanogenesis via protein kinase C-independent pathways.

Abstract:

Arbutin, a naturally occurring beta-D-glucopyranoside of hydroquinone, is effective in the topical treatment of various cutaneous hyperpigmentations characterized by hyperactive melanocyte function. We examined the mechanism of its depigmenting action in human melanocyte cultures. Arbutin inhibited the tyrosinase activity of cultured human melanocytes at noncytotoxic concentrations. It did not affect the expression of tyrosinase mRNA. Melanin production was inhibited significantly by arbutin, as determined by measuring eumelanin radicals with an electron spin resonance spectrometer. The study of the kinetics and mechanism for inhibition of tyrosinase confirms the reversibility of arbutin as a competitive inhibitor of this enzyme. The utilization of L-tyrosine or L-dopa as the substrate suggests a mechanism involving competition with arbutin for the L-tyrosine binding site at the active site of tyrosinase. These results suggest that the depigmenting mechanism of arbutin in humans involves inhibition of melanosomal tyrosinase activity, rather than suppression of the expression and synthesis of tyrosinase.

Abstract:

Novel hybrid L-ascorbic acid (vitamin C) derivatives with other biologically active substances, 5-hydroxy-2-hydroxymethyl-beta-pyrone (kojic acid) and alpha-tocopherol (vitamin E), linked at the C-2 or C-3 hydroxyl group were synthesized, and their thermal stability and inhibitory effects on tyrosinase activity, active oxygen species (AOS), and free radicals were estimated in vitro. It was found that a hydrophilic derivative, 2-O-(5-hydroxy-4H-pyran-4-one-2-methyl)-L-ascorbic acid (1), exhibited good thermal stability and inhibitory activities against tyrosinase catalyzed melanin formation, AOS, and free radicals compared to vitamin C and its conventional derivatives (such as the 2-phosphate 6-stearate and 2.6-dipalmitate, and 2-O-octadecylascorbic acid), as well as vitamin E, kojic acid, and arbutin. It is apparent that 1 has the biological properties of vitamin C and kojic acid, and acts synergistically. The hydroxyl groups at the C-3 position of the vitamin C moiety and the C-5 position of the kojic acid moiety are critical for the biological activities. We consider that the kojic acid moiety of 1 counterbalances the diminution of the biological activity due to shielding of the biologically important C-2 hydroxyl group of the vitamin C moiety. In addition, the thermal stability was significantly improved relative to not only vitamin C but also kojic acid. Further, a lipophilic derivative, 3-O-[(alpha-tocopheryloxy)-2-hydroxypropyl]-L-ascorbic acid, 2, was far more stable than vitamin C and its typical lipophilic derivatives. Compound 2 exhibited almost the same inhibitory activities against tyrosinase-catalyzed melanin formation, AOS, and free radicals as typical lipophilic derivatives, although these biological activities of 2 were lower than those of vitamin C.

Abstract:

The inhibitory effect of 50% ethanol extracts obtained from the leaves of six Arctostaphylos plants (A. patula, A. viscida, A. canescens, A. columbiana, A. nevadensis and A. uva-ursi) (Ericaceae) was tested on melanin biosynthesis. Among them, the extracts of A. patula and A. viscida showed potent inhibition against tyrosinase. The two extracts not only inhibited the production of melanin from dopachrome by autoxidation, but exhibited SOD (superoxide dismutase)-like activity and had a moderate absorbance in the UV-B area. These results suggest that the leaves of Arctostaphylos plants, especially A. patula and A. viscida can be applied to a whitening agent for the skin.

Abstract:

Glutathione (GSH) performs several important biological functions, including quenching of reactive oxygen species, and protection of cells from toxic compounds such as quinones. The first step in the synthesis of GSH is catalysed by gamma-glutamylcysteine synthetase, an enzyme which is inhibited by cystamine and buthionine sulfoximine (BSO). In this study, we examined the possibility that the effect of hydroquinone (HQ) on pigmentation could be potentiated by inhibiting the production of GSH. In vitro studies using melanoma cell lines demonstrated that both cystamine and BSO could potentiate the inhibitory effects of HQ on tyrosinase activity and melanin content. A synergistic decrease in hair pigmentation was observed when a combination of HQ (2 or 4%) and BSO (5%) was applied to the dorsal skin of C57BL mice. In black hairless guinea-pigs, the application of HQ plus either BSO or cystamine resulted in a significant decrease in epidermal pigmentation when compared with any of the agents alone. The possibility exists that in the future a combination of HQ plus cystamine or BSO could be used to treat disorders such as melasma and post-inflammatory hyperpigmentation.

Abstract:

In previous studies we have shown melanotic melanomas to be exquisitely more sensitive to hydroquinone (HQ) inhibition than non-melanotic cell lines in vitro. Indeed, incorporation of [H3] Urd and [H3] Thd have been shown to be respectively 80 and 35 times more sensitive to HQ inhibition. The difference between the cell lines studied was their derivation, marked by their different melanin contents. The presence of melanin was proposed as a possible explanation of the differences. However, comparative experiments reported here demonstrate that amelanotic melanoma cell lines are equally susceptible to HQ inhibition. Thus, the action of HQ is apparently independent of the melanin content of the cell. Significantly, the tyrosinase levels in the melanomas and the amelanomas were found to be comparable and markedly different from that in the non-melanoma control cell lines. Thus, the results reported here support the hypothesis put forward by other workers that hydroquinone melanotoxicity is independent of cellular melanin content but requires the presence of active tyrosinase.

Abstract:

Melanocyte stimulating hormone (MSH) specifically induces differentiation of mammalian melanocytes. To further define the biochemical events elicited by this stimulus, we have cloned murine melanoma cells which are either highly responsive or nonresponsive to MSH, and have examined their ultrastructural appearance, their melanogenic activities, and also their expression of tyrosinase. We have found that the basal levels of melanogenic activity in pigmented and nonpigmented cells correlate with expression of surface MSH receptors rather than with production of tyrosinase. Nonpigmented cells produce a potent, highly stable inhibitor of melanogenesis; this inhibitor acts directly on tyrosinase to dramatically and abruptly suppress melanin production. This posttranslational control of tyrosinase activity may represent a critical regulatory point in mammalian pigmentation.